Electrochemical Corrosion Behavior and Mechanism of (Alx)Mo0.8NbTiyW0.2Zr Refractory High-Entropy Alloys with Different Al Contents in NaCl Solution

Refractory high-entropy alloys (RHEAs) are promising candidates for next-generation high-temperature structural applications because of their excellent high-temperature mechanical properties and environmental stability. However, in marine power systems and high-salinity environments, chloride-induced localized corrosion remains a key factor limiting their application. Although Al alloying is effective for weight reduction and microstructural regulation, its effects on the passivation behavior and corrosion mechanism of RHEAs remain unclear. In this work, the electrochemical corrosion behavior of (Alx)Mo0.8NbTiyW0.2Zr RHEAs with different Al contents in 3.5 wt.% NaCl solution was investigated, together with the corresponding post-corrosion microstructural evolution. All alloys exhibited clear passivation during potentiodynamic polarization, and the combined electrochemical results and thermodynamic considerations suggest that Nb-containing oxides may contribute to passivation in this alloy system. As the Al content increased, the passivation potential changed only slightly, whereas the passive-film breakdown-related potential decreased markedly, indicating reduced passive film stability. Microstructural analysis showed that Al-for-Ti substitution was associated with the formation of Al-Zr-rich compounds and an increase in their measured area fraction. The preferential dissolution of the Al-Zr-rich compounds during anodic polarization indicates their greater local corrosion susceptibility under the applied test conditions, while their spatial distribution suggests a contribution from microstructural electrochemical heterogeneity. Thus, the corrosion behavior of this alloy system in chloride-containing environments may reflect the combined effects of passivation, potentially involving Nb-containing oxides, and local electrochemical heterogeneity associated with the coupled increase in Al and decrease in Ti. Since Al and Ti contents are coupled (x + y = 2), the observed microstructural evolution and corrosion degradation cannot be attributed solely to Al, but rather to the synergistic effect of Al increment and Ti decrement.